(1) Field of the Invention
The present invention relates to a method for fabricating a semiconductor device in which a semiconductor memory element having an ONO (silicon oxide layer/silicon nitride layer/silicon oxide layer) film as a gate insulating film and a semiconductor element other than the memory element are formed on a semiconductor substrate.
(2) Description of Related Art
As semiconductor memories, flash memories are now commonly used which are floating gate MOS transistors. On the other hand, memory devices have recently come to be used which have no floating gate and use ONO films (Oxide-Nitride-Oxide) as gate insulating films (see, for example, Japanese Unexamined Patent Publication No. 2001-77220).
Gate insulating films of memory devices using ONO films each have a three-layer structure with a silicon nitride film interposed between two silicon oxide films. Such memory devices hold data by storing charge in silicon nitride films. Unlike floating gate flash memories, the memory devices do not lose every charge even with defects existing in silicon oxide films, because charge is stored in silicon nitride films. Therefore, nonvolatile memories with excellent reliability can be constructed. Furthermore, such memory devices are excellent in terms of cost because of the absence of floating gates.
Furthermore, in recent years, memory devices using ONO films have also been suggested to have a multi-valued memory structure for storing multibit data in a single memory cell by storing charge in a plurality of parts of a silicon nitride film of the memory cell. Moreover, not only such an advantage but also the simplicity of this structure reduce the number of necessary masks used in additional processes even in the application of a memory device to an SoC (System on Chip) in which a plurality of devices are combined on a single chip. Therefore, memory devices using ONO films are advantageous as compared with floating gate flash memories.